Load distribution hitches-an analysis

[RichardT]

Originally Posted by 59toaster

U b a smart feller.....

What if you switched the "sm" and the "f"

[59toaster]

Originally Posted by RichardT

What if you switched the "sm" and the "f"

He would have to chang his nick to "stinky" then.

[2airishuman]

hi nick and others

this is a good effort and explanation of what load bars do and how much (proportional weight) force is required to redistribute tongue weight... redistributing tongue load takes less bar deflection with short trucks and trailers....right?

you must me in mathematics nick?

it always gets "deep" when words are combined with math forumla...i've always thought graphics with math made for better understanding.

my background is medicine, so i only understand enough math and physics to kill folks...

now for a couple of issues/questions....i agree the trailer can function like a simple seesaw (single fulcrum lever) but the truck? wouldn't we need to know the center of gravity/balance point for the truck? most of the diesel rigs are very front heavy...until we get the beds loaded up....but still seems we'd need to know cog instead of just the overhand?

next the load bars are steel with a stiffness/deflection/elasticity that isn't linear...what i'm trying to say is that as the bars bend the force needed to bend them changes....although your example does focus on lbs of force not inches of flex...but as the bars flex the 'effectiveness' of their force transfer changes...? increasing force from 1000 to 1400 doesn't really move 1/3 more does it?

and wouldn't you really need an inline tensiometer on the chains/worm drives to measure the lbs of force generated in the load bars?

so would i use a spreadsheet to set up my rig....not at this point. i think there are too many variables not accounted for, for the math to be accurate....the cat scales would still seem a better method.

lastly...even if you finesse the math some this still doesn't help those folks trying to understand inland andy's issue of shaking the trailers (and nick isn't trying to explain this)...........

inland andy's concept of light bars on stiff trucks.... is really about trying to decrease transmission of high frequency/low amplitude vibration....i think.

cheers
2air'

[fastrob]

Very nice thought chain. Good work. Thanks for sharing.

I called the manufacturer and asked enough questions until satisfied about the answers for our setup.

But there is more to it than this; nothing can compensate for an incompetent driver. Happy Trails.

[CanoeStream]

C'mon y'all -- if you hadn't been dear hunting last weekend you'd have seen this post that probably was the final stimulus to get Nick thinking: http://www.airforums.com/forum...rty-19133.html

What we really need is more dialog at the level 59toaster & 2airishuman are suggesting. It's not easy to understand but there are some tough choices to analyze. Nick, I'm following so far. Thanks!

[Pick]

Loved that post Nick. What would really be interesting, is if we could find some electronic tension sensors, similar to what they use to convert old beam scales to electronic scales. Replace a link in each chain with the sensors, and actually measure the tension. Then drive down the road and watch the chain tension change. Of course we would have to have a laptop recording and graphing all this........

[nickcrowhurst]

To respond to some of the comments so far:
Markdoane said
“If someone would enter the spring rates (lbs/in deflection) for the various weight bars, we could combine this with Inland Andy's recommendation for 1-2 inch bend and tell exactly which bars to order.”

Yes, exactly so, and a great idea. The equations are simple practical tools to assist in the selection of a load distribution hitch. An analogy would be the decision as to which paddle a kayaker should buy. The vendor should give advice based on the height of the paddler, the beam of the kayak, the type of water being traveled. The kayaker should then be equipped with a paddle “fit for purpose”. However, it doesn’t mean she’ll be safe or efficient in the real world situation of moving water. That will require skill, experience and judgment. Just like towing heavy.

59 toaster said
“That will once and for all put it to rest what bars with how much deflection is needed to make a DC work properly in an emergency situation and back up Inland Andy's observations about "over hitched".

My guess (and it’s no more than that, so not very helpful), is that although a certain bar tension is required for a given rig for correct load distribution, this tension could be obtained from either:
(a) A stiff bar with a small deflection, OR
(b) A soft bar with a larger deflection.

I guess that the soft bar with the larger deflection would be far preferable when the rig is traveling. When traveling over bumps, the load distribution effect would remain steadier, as a one inch deflection of the soft bars would have less impact on this effect and hence the trailer behavior would be more stable. Also, the impact loads on the trailer would be lessened, and thus cause less stress on the trailer. (Andy’s point, I believe). As 59 toaster said, this will enable more consistent sway control from the Dual Cam hitches.

2Air makes some very good points
“....i agree the trailer can function like a simple seesaw (single fulcrum lever) but the truck? wouldn't we need to know the center of gravity/balance point for the truck? most of the diesel rigs are very front heavy...until we get the beds loaded up....but still seems we'd need to know cog instead of just the overhang?”

2Air has hit on the central difficulty in analyzing this issue. In the real world, the center of rotation of the TV will depend on the relative hardness of the front and rear springs. In the past I have attempted to include these variables, but the math became too complex to be of practical use to me. By assuming the suspension is rigid, I have attempted to produce a practical, usable, set of equations whose accuracy is sufficient for the purpose. By concentrating on returning the TV front axle to its original height, I have attempted to minimize the distortion caused by the assumption.

“increasing force from 1000 to 1400 doesn't really move 1/3 more does it?”

Yes, I believe so. As you correctly state, I have concentrated on the tension in the chains, and not the required deflection of the bars. The sophisticated way to find out is by following Pick’s excellent suggestion!

I have to take my 4 year old granddaughter (number 5) out into the woods to show her how to break trail, mark trail etc, so farewell for now.
Nick.

[SafeHarbor]

What I'm very happy about is that I also have an Excella 31 and a 3/4 ton Dodge diesel. Now that Nick's done the thinking... I'll just get whatever lift bars he gets.

Lamar

[nickcrowhurst]

Lamar, I've just entered your figures into the equation, assuming your empty tongue weight of 580 pounds, 33 inch bars, and 206 inches (a guess) from trailer axle center to ball, and rear TV overhang of 60 inches. The chain tension required is 816 pounds, or 408 pounds per chain. That's about 20% less than my rig, because of your lower tongue weight I obtained from searching this forum. I use an Eaz-lift 1000lb hitch!
Nick.

[SafeHarbor]

Thanks, Nick!

I've just read the thread from last year that contains lots of good information from Inland Andy and others, and my fuzziness on this is clearing up somewhat.

The purpose of using the lighter weight lift bars is to acheive more deflection and springiness (boingo-boingo) in the lift bars while maintaining enough force to keep the bolt-on (in my case) cams seated in straight-ahead motion, bumpy motion, and stopping motion. This seating pressure is also essential to maintaining the antisway properties.

A too-heavy set of lift bars does not have enough springiness, so the shearing forces transmitted to the front end of the trailer eventually beat the trailer to death.

But... here's where I'm still fuzzy. Can the 550 lb lift bars distribute more than 550 lbs tounge weight to the TV and AS? Since there are two of them, does that mean they can distribute 550 lbs x 2? (See, I'm capable of making my OWN head hurt.)

Remember that started me out with all of this are the 1,000 lb lift-bars I have that worked GREAT on my Dakota.

Lamar

[nickcrowhurst]

Originally Posted by SafeHarbor

Can the 550 lb lift bars distribute more than 550 lbs tounge weight to the TV and AS? Since there are two of them, does that mean they can distribute 550 lbs x 2?Lamar

Only the manufacturer can explain what is meant by their ratings. The Eaz-Lift website describes a 1000 pound hitch as being able to redistribute a 1000 pound tongue weight. This is, at best, a simplification based on the "average TV rear overhang" and " average hitch to trailer axle center distance". Certainly, the chain tension is doubled if two chains are under equal tension.To answer your previous point, from equation 8,
D= (T*H*L)/(B*(H+L)) hence
D/T=(H*L)/(B*(H+L)) This shows that a total chain tension can distribute a tongue load greater than that chain tension, if (B*(H+L) is greater than (H*L).
Nick.

[the ponz]

Please recalculate using the following additional variables:


40,000# tractor trailer...



applied to the rear bumber.



BarkingSpider, did you replace the hitch along with the trailer?

[Silvertwinkie]

Originally Posted by the ponz

It would seem to me that there must be an easier way for one to get to 500 posts

Nick, even though you lost me mid way, I love the fact that you've thought this that far through.

As for what ponz said about easier ways to get to 500 posts, heck, you should see some the post I have that took to get to 4600 and change this one had far more substance (and assumptions too)!

[nickcrowhurst]

Originally Posted by Silvertwinkie

Nick, even though you lost me mid way, I love the fact that you've thought this that far through.

Twink, thank you for the kind encouragement. If I've lost you, that's a failing in my explanation.
Nick.